The present invention relates to a method for concurrently displaying two video images derived from respective asynchronous sources and to memory efficient circuitry for implementing the method.
In a television receiver having picture-in-a-picture (pix-in-pix) capability, a small video image derived from an auxiliary source is displayed as an inset in a full size image derived from a main source. The main and auxiliary sources may be, for example, two independent broadcast television signals. Since these signals are independent, they are likely to be asynchronous. That is to say, there may be phase and frequency differences in the horizontal line timing and vertical field timing of the two signals as well as in their chrominance subcarrier signals.
These timing differences between the main and auxiliary signal may cause distortions in the auxiliary image when it is displayed using timing signals derived from the main image. Image distortion caused by the different chrominance signal phases appear as erroneous or changing colors in the reproduced image. This type of distortion is generally overcome by independently demodulating the chrominance signal components of the main and auxiliary video signals. Distortion caused by the different horizontal line and vertical field scanning signal phases may be overcome by sampling the auxiliary signal in synchronism with timing signals for the auxiliary image and then displaying the stored samples in synchronism with timing signals for the main image.
A particular type of distortion which is of interest in this application occurs when the field scanning signals of the main and auxiliary video signals are not aligned. FIGS. 1a through 1c illustrate this type of distortion. FIG. 1a represents a video image of a diamond-shaped form as reproduced on a conventional interlace scan display. In an interlace scan display, each frame of an image is composed of two time-sequential interlaced field images, an upper field image and a lower field image. In FIG. 1a, the circles represent pixels of an upper field and the crosses represent pixels of a lower field. The terms "upper" and "lower" are used to describe the two fields to emphasize that, for proper reconstruction of the image, each line of the lower field should be displayed below a corresponding line of the upper field. In FIG. 1a, the upper field line numbers all have a suffix of U and the lower field line numbers all have a suffix of L.
FIG. 1b represents a vertical compression of the image shown in FIG. 1a in a ratio of three to one. In a conventional pix-in-pix system, the inset image is compressed both horizontally and vertically. However, to avoid unnecessary confusion, the images shown in FIGS. 1b, 1c and 3 are only vertically compressed. The image shown in FIG. 1b represents a vertical compression of the image when two small picture fields are stored in a memory and displayed in proper alignment with the main picture. That is to say, when the upper and lower fields of the compressed auxiliary image are displayed as parts of the upper and lower fields of the main image, respectively.
If the memory includes storage space for only one field of video samples, an image such as that shown in FIG. 1b is produced only when the vertical field synchronizing signals components of the two signals are properly aligned. In this alignment, one complete field of the auxiliary signal is received and stored before the small picture is displayed and the stored field is of the same type (upper or lower) as the displayed main field. More commonly, the main and auxiliary signals are aligned such that, when the compressed image is displayed, the stored auxiliary signal includes portions of two successive fields, one upper field and one lower field. This image may be distorted because, in a portion of the image, the relative positions of the upper and lower fields are reversed.
An misalignment of this type occurs when the compressed auxiliary image is being read from the memory for display at the same time that new data, to be displayed in the next field period, is being written into the memory. If, for example, the read-write overlap for the memory occurs at line 5 of the auxiliary image, as indicated by the broken line in FIG. 1a, a distorted compressed image, such as that shown in FIG. 1c may be produced. In this image, the lower field lines 7, 10, 13 and 16 are erroneously displayed in positions above the upper field lines 6, 9, 12 and 15, distorting the lower portion of the compressed image.